The present invention is directed to a method for sensing the denomination of a currency. More specifically, the invention contemplates verifying, for example, a one dollar denomination of U.S. currency versus all of the other dollar denominations and in addition, in a different mode of operation selecting several different denominations from a group of currency bills.
Existing denomination sensors rely on several different techniques. In general some characteristic of the denomination is sensed and compared with a reference standard. In Mustert U.S. Pat. No. 3,679,314 different spectral distributions of the bill are sensed; in Carter U.S. Pat. No. 3,870,629 phase locked loops are used to detect frequency characteristics of the bills. In Riddle U.S. Pat. No. 3,280,974 changes in the magnetic flux of a moving bill are sensed due to spatial variation of the magnetic printing ink; finally, in Hong U.S. Pat. No. 3,845,466 the output of a photodetector is processed to form a probability density function which is compared with a prestored function.
All of the foregoing techniques are either excessively complicated for high speed verification or else lack the required accuracy; i.e., accuracies of less than 99% are not acceptable.